A variety of chromium containing aqueous solutions have heretofore been used or proposed for treating zinc, zinc alloy, cadmium, cadmium alloy and aluminum surfaces for improving the corrosion resistance properties thereof. Such treating solutions originally contained chromium in the hexavalent state and in more recent years the chromium constituent was present as a mixture of the hexavalent and trivalent forms. The reduced toxicity of trivalent chromium and the increased simplicity and efficiency in treating waste effluents containing trivalent chromium has occasioned an increased commercial use of passivate solutions in which the chromium constituent is substantially entirely in the trivalent state. The present invention is directed to a passivating solution which does not contain any chromium ions and is effective for imparting corrosion resistance to zinc, cadmium and aluminum surfaces as well as alloys thereof.
Typical of prior art chromium-free compositions and processes for treating metal surfaces are those disclosed in U.S. Pat. Nos. 3,539,402; 3,539,403; 3,586,543; 3,682,713; 3,687,740 and 3,843,430.
The present invention provides a treating solution and process which is effective to selectively impart a clear blue-bright or a clear light-yellow passivate film to zinc, zinc alloy, cadmium, cadmium alloy, aluminum and magnesium surfaces which provides for improved corrosion resistance. The present invention is further characterized by a process which is simple to control and operate and which is of efficient and economical operation.